1. Field of the Invention
This invention relates to a process for preparing an alkylhalosilane by direct synthesis, and more particularly to a continuous process for preparing an alkylhalosilane by effecting gas-solid contact reaction between metallic silicon and alkyl halide in the presence of a copper catalyst.
2. Prior Art
With respect to the synthesis of alkylhalosilanes, Rochow first disclosed in U.S. Pat. No. 2,380,995 direct synthesis reaction between metallic silicon and an alkyl halide in the presence of a copper catalyst. Since then, there have been reported a number of research works relating to various co-catalysts used together with copper catalysts, various copper catalysts and treatment thereof, reactors, additives used during reaction, and the like.
The direct synthesis process involves activating a contact mass comprising metallic silicon and a copper catalyst, and introducing an alkyl halide into the activated contact mass for accomplishing gas-solid direct contact between metallic silicon and alkyl halide, thereby producing alkylhalosilanes.
In the industrial synthesis of alkylhalosilanes, the selectivity of dialkyldihalosilane and the formation rate of silanes are crucial because the dialkyldihalosilane is used most often in silicone resins. The selectivity of dialkyldihalosilane is evaluated in terms of a weight or molar ratio of dialkyldihalosilane to the silanes produced and a T/D ratio. Alkylhalosilane products contain dialkyldihalosilane (D), trialkylhalosilane (M), alkyltrihalosilane (T), etc. as well as other by-products such as alkylhydrodihalosilane (H) and alkylhalodisilane. In particular, disilanes are known as a residue or waste among silane manufacturers because few processes are available for the effective utilization of disilanes, and most disilanes are discarded. The T/D ratio is a compositional ratio of alkyltrihalosilane to dialkyldihalosilane in the entire alkylhalosilanes produced, with a lower T/D ratio being preferred. The formation rate of alkylhalosilane is represented by a space time yield (STY) which is the weight of crude alkylhalosilanes produced per unit time relative to the weight of metallic silicon held in the reactor. In order to improve the content of dialkyldihalosilane produced, reduce the T/D ratio or increase the STY, various research works have been made with a focus on the catalyst and accelerator.
USSR Application Specification No. 617,569 (Certificate of inventorship No. 122,749) dated Jan. 24, 1995 discloses reaction in the presence of a metallic silicon-copper alloy with 20 to 40 ppm of antimony added. Allegedly, the dimethyldichlorosilane content is improved from 40% to 60%. U.S. Pat. No. 4,500,724 discloses use of a copper/zinc/tin catalyst containing 200 to 3,000 ppm of tin, thereby achieving an improvement of T/D to 0.037. Japanese Patent Publication (JP-B) No. 92421/1994 discloses reaction using copper arsenide having an arsenic concentration of at least 50 ppm. It is described in these patent references that reactivity, more specifically the rate of reaction of metallic silicon is improved by adding these tin, antimony and arsenic co-catalysts to a reaction contact mass comprising metallic silicon and copper.
USSR Application Specification No. 903,369 (Certificate of inventorship No. 178,817) dated Jun. 2, 1964 discloses that a co-catalyst selected from the group consisting of zinc, bismuth, phosphorus (200 ppm), arsenic, tin, and iron improves the dimethyldichlorosilane content to 72.1% from the value achieved by the above-referred Application Specification No. 617,569 (Certificate of inventorship No. 122,749). Also USSR Application Specification No. 1,152,943 (Certificate of inventorship No. 237,892) dated Nov. 20, 1969 discloses to add a phosphorus-copper-silicon alloy to a contact mass so as to give 2,500 to 30,000 ppm of phosphorus, thereby improving the dimethyldichlorosilane content to 82.3%. Moreover, U.S. Pat. No. 4,602,101 corresponding to JP-B 51596/1993 discloses that 25 to 2,500 ppm of a phosphorus compound capable of generating elemental phosphorus in the reactor is added to a contact mass.
For improving the direct synthesis of alkylhalosilanes, a number of researchers have made investigations on various metal co-catalysts. There is a desire to have more effective co-catalysts.